What factors affect the cycle life of lithium battery packs? Lithium batteries are a new type of battery that has gained widespread acceptance in recent years for its ability to extend battery life. However, after multiple charge-discharge cycles, battery capacity and other performance characteristics will decline. Under the same conditions, the faster the battery capacity decays, the relatively worse the battery quality. The cycle performance of a lithium battery pack is an important indicator of its quality, and many standards for lithium battery packs include cycle life as a metric.
The charge-discharge cycle of a lithium battery pack is a complex physicochemical reaction process, and its cycle life is affected by many factors. The following is an analysis of the factors influencing the cycle life of lithium battery packs.
1. Design and manufacturing process
Material selection is the most crucial factor in battery design. Different materials exhibit different performance characteristics, leading to variations in battery performance. Good cycle performance is achieved through proper matching of positive and negative electrode materials, resulting in a longer battery cycle life. Generally, the design and assembly process requires a slight excess of negative electrode capacity compared to the positive electrode. If this excess is not achieved, lithium will precipitate from the negative electrode during charging, forming lithium dendrites and compromising safety. Conversely, if the negative electrode capacity is excessively high compared to the positive electrode, excessive lithium delithiation from the positive electrode may occur, causing structural collapse.
The type and amount of electrolyte also affect battery life. The manufacturing process of lithium battery packs mainly includes: positive and negative electrode material preparation, coating, sheet forming, winding, casing, electrolyte injection, sealing, and formation. Each step in the battery production process requires very strict control. Failure to properly control any step can potentially affect the battery's cycle performance.
2. Aging and degradation of lithium battery materials
The charge-discharge cycle of a lithium-ion battery pack is the process of lithium ions moving back and forth between the positive and negative electrode materials through the electrolyte. During the cycle, in addition to the redox reactions occurring at the positive and negative electrodes, numerous side reactions also occur. If these side reactions can be reduced to a low level, ensuring that lithium ions can smoothly move between the positive and negative electrode materials through the electrolyte, the cycle life of the lithium-ion battery can be increased.
The properties of the current collectors at the positive and negative electrodes also affect the battery's capacity and cycle life. The commonly used current collector materials for the positive and negative electrodes of lithium-ion battery packs are aluminum and copper, respectively, both of which are easily corroded metals. Corrosion of the current collector, resulting in a passivation film, poor adhesion, localized corrosion (pitting corrosion), and generalized corrosion, all increase the battery's internal resistance, leading to capacity loss and reduced discharge efficiency. Pretreatment methods such as acid-alkali etching and conductive coating can enhance its adhesion and corrosion resistance.
3. Charge and discharge regime during the cycle
The use of a lithium-ion battery pack involves a charge-discharge cycle. The magnitude of the charge-discharge current, the selection of the charge-discharge cutoff voltage, and the charge-discharge method used all significantly impact the cycle life of the lithium-ion battery. Blindly increasing the battery's operating current, raising the charging cutoff voltage, or lowering the discharging cutoff voltage will all degrade the performance of the lithium-ion battery pack.
Different electrochemical systems of lithium-ion batteries have different charge/discharge cutoff voltages. During the charging process of a lithium-ion battery, exceeding the charging cutoff voltage is considered overcharging. When a lithium-ion battery is overcharged, excess lithium ions released from the positive electrode will deposit or embed into the negative electrode. The deposited active lithium readily reacts with the solvent, releasing heat and raising the battery temperature. When the discharge voltage of a lithium battery falls below the discharge cutoff voltage, over-discharge occurs.
During over-discharge, lithium ions excessively escape from the negative electrode, making re-entry during the next charge more difficult. After over-discharge, the discharge capacity and charge/discharge efficiency of a lithium battery are significantly reduced during cycling. Furthermore, lithium batteries are highly susceptible to melting under high current conditions, and equipment components may also be damaged.
4. Lithium battery operating environment
The operating environment of lithium battery packs has a significant impact on their cycle life. Ambient temperature is a particularly important factor; both excessively low and high temperatures can negatively affect the cycle life of lithium batteries.
The standard operating temperature for lithium batteries is -20°C to 60°C. However, below 0°C, the performance of lithium batteries generally declines, and their discharge capacity decreases accordingly. Therefore, the optimal operating temperature for lithium batteries is typically 0°C to 40°C. Temperature requirements for lithium batteries in special environments may vary. Additionally, ensure good ventilation to facilitate heat dissipation and maintain a clean environment.
At high temperatures, the charge-discharge cycle of lithium-ion batteries is unstable. High temperatures intensify electrochemical polarization of the battery electrodes and generate gas, causing swelling. Simultaneously, charge transport resistance increases, and ion transport kinetics deteriorates. At low temperatures, constant-voltage charging time increases, and charging performance also deteriorates significantly. Devices using lithium-ion batteries may be subjected to vibration, shock, and collisions during transportation or normal operation. Some lithium batteries charge and discharge and receive data at specific frequencies when communicating with systems.
The cycle life of lithium battery packs is influenced by many factors. The increasingly widespread application of lithium-ion batteries has placed higher demands on both the quantity and quality of these batteries. Cycle life directly affects the usage time and quality of lithium batteries; therefore, it is essential for manufacturers to study the influencing factors.
